This invention relates to a solid state imaging device, a signal processing method and a driving method therefor and a camera, and more particularly to an X-Y address type solid state imaging device as represented by an amplification type solid state imaging device and a signal processing method for the solid state imaging device as well as a camera which employs an X-Y address type solid state imaging device as an imaging device. The present invention further relates to an amplification type solid state imaging device wherein pixels themselves have an amplification function and signals of the pixels are outputted as voltages and a driving method for the amplification type solid state imaging device.
An X-Y address type solid state imaging device includes, as shown in FIG. 14, a pixel section 111 wherein a large number of pixels are arranged in rows and columns, a vertical scanning circuit 112 for successively selecting the rows of the pixel section 111, a horizontal scanning circuit 113 for successively selecting the columns of the pixel section 111, and an output circuit (charge detection circuit) 114 for outputting a signal. The vertical scanning circuit 112 and the horizontal scanning circuit 113 are each formed from, for example, a shift register and successively generate a vertical scanning (vertical selection) pulse signal xcfx86V and a horizontal scanning pulse signal xcfx86H one by one for each row and each column, respectively.
In this X-Y address type solid state imaging device, when the horizontal scanning circuit 113 holds signal charge of the pixels in capacitors and outputs the pixel signals of the capacitors to the output circuit 114 from a horizontal signal line via horizontal switches each formed from a MOS transistor, dispersions in threshold voltage Vth of the horizontal switches (MOS transistors) are superposed on the pixel signals, and they appear as vertical string-like fixed pattern noises on a screen, deteriorating the picture quality.
Here, a generation mechanism of vertical string-like fixed pattern noises generated from the horizontal scanning circuit 113 is described with reference to an equivalent circuit of a signal path for the nth column of the horizontal scanning circuit 113 shown in FIG. 15. As a presupposition of description, notice is taken here of a dispersion in threshold voltage Vth of a horizontal switch.
Referring to FIG. 15, a pixel signal held by a capacitor 121 flows as charge to a horizontal signal line 123 when a horizontal scanning pulse xcfx86Hn is supplied from a horizontal shift register (not shown) to the gate electrode of a horizontal switch 122 so that the horizontal switch 122 is put into a connecting state, and is then demodulated into a voltage by and outputted from an output circuit 124. In this instance, if the threshold voltage Vth which defines the boundary between a disconnecting state and a connecting state of the horizontal switch 122 exhibits a dispersion for each of the horizontal switches of the individual columns, then charge represented by the product of the dispersion in threshold voltage Vth and a variation amount in capacitance generated between the horizontal switch 122 and the horizontal signal line 123 appears on the horizontal signal line 123. Therefore, a vertical string-like fixed pattern noise which corresponds to the charge amount is superposed with the pixel signal.
The manner of such superposition is described with reference to FIGS. 16A and 16B which show equivalent circuits where the horizontal switch 122 is converted into a capacitance model. More particularly, FIG. 16A shows an equivalent circuit where the horizontal switch (MOS transistor) 122 is in a disconnecting (off) state, and FIG. 16B shows an equivalent circuit where the horizontal switch (MOS transistor) 122 is in a connecting (on) state.
In FIG. 16A, the horizontal switch 122 is in a disconnecting state and, as a capacitance model, a gate-drain capacitance 122a is produced between the gate electrode of a horizontal switch (MOS transistor) to which a horizontal scanning pulse xcfx86Hn is applied and the capacitor 121 by which a pixel signal is held while a gate-source capacitance 122b is produced between the gate electrode of the horizontal switch and the horizontal signal line 123, and the capacitor 121 and the horizontal signal line 123 are disconnected from each other.
On the other hand, in FIG. 16B, the horizontal switch 122 is in a connecting state and the capacitor 121 is connected to the horizontal signal line 123, and a gate-channel capacitance 122c is produced between the gate electrode of the horizontal switch (MOS transistor) to which a horizontal scanning pulse xcfx86Hn is applied and the horizontal signal line 123. Here, the capacitance of the gate-channel capacitance 122c is considerably higher than the total capacitance of the capacitance 122a and the capacitance 122b. 
Since the two states of FIGS. 16A and 16B are changed over with reference to the threshold voltage Vth of the horizontal switch 122 by the voltage of the horizontal scanning pulse xcfx86Hn applied to the gate electrode of the horizontal switch 122, if the horizontal switch of each column has a dispersion in threshold voltage Vth, then the product of the dispersion in threshold voltage Vth and the difference between the capacitances of the horizontal switch 122 in the two states of FIGS. 16A and 16B appears as dispersion charge on the horizontal signal line 123 and makes a vertical string-like fixed pattern noise.
Now, where the capacitances of the capacitance 122a, capacitance 122b and capacitance 122c are represented by Cdg, Cgs and Cg, respectively, the dispersion of the threshold voltage Vth of the horizontal switch 122 is represented by xcex94Vth, the dispersion charge appearing on the horizontal signal line 123 is represented by xcex94q, the capacitance of a detection capacitor 125 of the output circuit 124 is represented by Cd, and a vertical string-like fixed pattern noise appearing on the output is represented by xcex94Vout, the dispersion charge xcex94q and the fixed pattern noise xcex94Vout are given by
xcex94q=(Cgxe2x88x92Cgdxe2x88x92Cgs)xc2x7xcex94Vth
xcex94Vout=xcex94q/Cd
Particularly, giving an example of numerical values, if Cgd and Cgs are 1 fF, Cg is 20 fF, the dispersion xcex94Vth of the threshold voltage Vth is 50 mV and the capacitance Cd of the detection capacitor 125 is 0.5 pF, then the fixed pattern noise xcex94Vout is 1.8 mv.
Driving timings of the ordinary X-Y address type solid state imaging device and a manner in which vertical string-like fixed pattern noises appear are illustrated in a timing chart of FIG. 17. A vertical scanning pulse signal xcfx86V (xcfx86V1, . . . , xcfx86Vm, xcfx86Vm+1, . . . ) for selecting pixel elements of the same row successively rises for each horizontal blanking period, and an operation pulse signal xcfx86OP rises in synchronism with the vertical scanning pulse signal xcfx86V. The operation pulse signal xcfx86OP is applied to the gate electrode of an operation switch (not shown) formed from a MOS transistor for reading out a pixel signal to the capacitor 121.
As the operation pulse signal xcfx86OP rises, pixel signals of a selected row are read out into the capacitors 121. The pixel signals of the certain row held in the capacitors 121 are, when a horizontal image period is entered, read out from the output circuits 124 as the horizontal switches 122 are successive put into a connecting state when the horizontal scanning pulse signal xcfx86H (xcfx86H1, . . . , xcfx86Hn, xcfx86Hn+1, . . . ) outputted from the horizontal shift register successively rises.
In this instance, if it is assumed, for example, that an equal signal amount is outputted from all pixels and only the threshold voltages Vth of the horizontal switches 122 have individual dispersions, then as seen in the timing chart of FIG. 17, an output signal OUT does not exhibit an equal signal amount, but exhibits dispersions in threshold voltage Vth of the horizontal switches 122 superposed on the pixel signals. Then, the dispersions appear as vertical string-like fixed pattern noises on the screen, deteriorating the picture quality.
As a method of preventing deterioration of the picture quality arising from vertical string-like fixed pattern noises, a possible method is to extract only fixed pattern noise components, hold them as a reference signal for cancellation and subtract, in an ordinary imaging operation, the reference signal from signal outputs of the solid state imaging device to cancel the fixed pattern noises.
However, while, in the description of the generation mechanism of fixed pattern noises given above, a manner in which fixed pattern noises appear on an output signal in the condition that no incident light is received is described, if light is irradiated upon a central portion of the imaging area here, then signal components arising from the incident light are added to the fixed pattern noise components, and such an output signal waveform as indicated by OUT-L in FIG. 17 is obtained. This output signal cannot be used as a reference signal for cancellation.
In other words, in a conventional X-Y address type solid state imaging device, in order to cancel vertical string-like fixed pattern noises, the solid state imaging device must be shielded against incident light by some method so as to output only fixed pattern noise components as a reference for a correction signal. More particularly, such a mechanical operation that a cover is fitted on the lens of the camera or incident light is intercepted by a mechanical shutter is required. Such an operation is disadvantageous in terms of the price or minimization of a camera since it urges a person who operates the camera to perform a manual operation for cancellation of fixed pattern noises or requires a part which is not originally necessitated for a camera such as a mechanical shutter.
Meanwhile, as amplification type solid state imaging devices, a CMD (Charge Modulation Device), a BASIS (Base Stored Image Sensor), a BCMD (Bulk Charge Modulation Device) and so forth are known. In those amplification type solid state imaging devices, since pixels are formed using an active element of a MOS structure or the like in order to make the pixels themselves have an amplification function, a dispersion in characteristic (threshold value Vth and so forth) of an active element is superposed as it is on an image signal. Since the dispersion in characteristic has a fixed value for each pixel, it appears as a fixed pattern noise (FPN) on a screen.
An exemplary one of conventional amplification type solid state imaging devices constructed so as to remove fixed pattern noises arising from characteristic dispersions of pixels is shown in FIG. 24. Referring to FIG. 24, a large number of pixels 301 are arranged in rows and columns, and control input terminals of the pixels 301 are individually connected to vertical selection lines 302 in units of a row while output terminals of the pixels 301 are connected to vertical signal lines 303 in units of a column. Terminals of the vertical selection lines 302 on one side are connected to output terminals of a vertical scanning circuit 304 for the individual rows. The vertical scanning. circuit 304 is formed from a shift register or a like element and successively outputs a vertical scanning pulse signal xcfx86V (xcfx86V1, . . . , xcfx86Vm, xcfx86Vm+1, . . . ).
Connected to each of the vertical signal lines 303 are the drains of two sampling switches 305s and 305n each formed from an N-channel MOS transistor. An operation pulse signal xcfx86OPS for sampling a signal voltage in a bright state prior to pixel resetting outputted from a pixel 301 is applied to the gates of the sampling switches 305s. Meanwhile, another operation pulse signal xcfx86OPN for sampling a signal voltage in a dark state after pixel resetting outputted from a pixel 301 is applied to the gates of the sampling switches 305n. 
The sources of the sampling switches 305s and 305n are connected to terminals of two capacitors 306s and 306n on one side, respectively. The capacitors 306s and 306n are provided to hold a signal voltage in a bright state and a signal voltage in a dark state, respectively, while the other terminals of them are grounded commonly. The sources of the sampling switches 305s and 305n are further connected to the drains of two horizontal selection switches 307s and 307n formed from N-channel MOS transistors, respectively.
The sources of the horizontal selection switches 307s and 307n are connected to a horizontal signal line 308, and the gates of the horizontal selection switches 307s and 307n are connected to output terminals of a horizontal scanning circuit 309 for the individual columns. The horizontal scanning circuit 309 is formed from a shift register or a like element and outputs a horizontal scanning pulse signal xcfx86H ( . . . , xcfx86Hn, xcfx86Hn+1, . . . ) for successively turning on the horizontal selection switches 307s and the horizontal selection switches 307n for the individual columns. The horizontal signal line 308 is connected to an input terminal of a horizontal output circuit 310. An output terminal of the horizontal output circuit 310 is connected to an input terminal of a CDS (correlation double sam- pling) circuit 311.
Subsequently, circuit operation of the conventional apparatus having the construction described above for removing fixed pattern noises is described.
If a certain row is selected by vertical scanning by the vertical scanning circuit 304 in a horizontal blanking period, then signal voltages in a bright state prior to pixel resetting and signal voltages in a dark state after pixel resetting of the pixels 301 of the selected row are successively sampled by the sampling switches 305s and 305n and held by the capacitors 306s and 306n, respectively.
Then, in a horizontal effective period, when a certain column is selected by horizontal scanning by the horizontal scanning circuit 309 and the horizontal selection switches 307s and 307n of the selected column are successively turned on, the signal voltages in a bright state and the signal voltages in a dark state held in the capacitors 306s and 306n are successively read out into the horizontal signal line 308, respectively. Consequently, the signal voltages in a bright state and the signal voltages in a dark state are successively transmitted in units of a column on a time base by the horizontal signal line 308 and supplied to the CDS circuit 311 through the horizontal output circuit 310.
By the CDS circuit 311, correlation double sampling of the signal voltages in a bright state and the signal voltages in a dark state which successively appear on the time base is performed, and finite differences between them are calculated to cancel noise components. As a result, a signal from which fixed pattern noises arising from dispersions in characteristic such as a threshold voltage Vth for the pixels 301 have been removed is obtained.
However, with the conventional amplification type solid state imaging device described above, while fixed pattern noises arising from characteristic dispersions of the pixels 301 can be removed, since the flows of signals in a bright state and a dark state are different in the sample hold circuit between the vertical signal lines 303 and the horizontal signal line 308, if some components are superposed on a signal by the sample hold circuit, then those components remain also after the correlation double sampling by the CDS circuit 311.
What are present as components which are superposed by the sample hold circuit are distribution noises of the sampling switches 305s and 305n and so forth. Where those components are different between columns because of dispersions in circuit characteristic, also the components which remain after the correlation double sampling exhibit dispersions and appear as vertical string-like fixed pattern noises on the screen.
It is an object of the present invention to provide a solid state imaging device and a signal processing method therefor as well as a camera by which a reference signal for cancellation of vertical string-like fixed pattern noises can be obtained readily even if a person who operates a camera is not urged to perform a manual operation or without using a mechanical shutter.
It is another object of the present invention to provide a solid state imaging device and a driving method for the same by which not only fixed pattern noises arising from characteristic dispersions of pixels but also vertical string-like fixed pattern noises arising from dispersions in characteristic of circuits can be suppressed.
In order to attain the object described above, according to an aspect of the present invention, there is provided a solid state imaging device, comprising a plurality of pixels arranged in rows and columns, a vertical scanning circuit for controlling control electrodes of the pixels in the same rows connected commonly by vertical selection lines, a horizontal scanning circuit for successively outputting pixel signals outputted through vertical signal lines, are connected commonly to which main electrodes of the pixels of the same columns, in units of a row, and an output circuit for outputting the pixel signals from the horizontal scanning circuit to the outside, the solid state imaging device having a first operation mode in which the pixels are reset after the pixel signals are read out and a second operation mode in which the pixel signals are read out after the pixels are reset.
In the solid state imaging device, in the first operation mode, the pixels are reset after pixel signals are read out, and consequently, an ordinary imaging operation is performed. On the other hand, in the second operation mode, since pixel signals are read out after the pixels are reset, processing for obtaining a reference signal for cancellation of vertical string-like fixed pattern noises which are generated from the horizontal scanning circuit is performed irrespective of whether or not there is incident light.
With the solid state imaging device, since it is constructed such that it has, separately from an ordinary imaging mode, an operation mode wherein pixel signals are read out after pixels are reset, a reference signal for canceling vertical string-like fixed pattern noises generated from a horizontal scanning circuit and so forth can be obtained readily by varying the operation timing of a vertical scanning circuit irrespective of whether or not there is incident light.
According to another aspect of the present invention, there is provided a signal processing method for a solid state imaging device which includes a plurality of pixels arranged in rows and columns, a vertical scanning circuit for controlling control electrodes of the pixels in the same rows connected commonly by vertical selection lines, a horizontal scanning circuit for successively outputting pixel signals outputted through vertical signal lines, to which main electrodes of the pixels of the same columns, are connected commonly in units of a row, and an output circuit for outputting the pixels signals from the horizontal scanning circuit to the outside, and has a first operation mode in which the pixels are reset after the pixel signals are read out and a second operation mode in which the pixel signals are read out after the pixels are reset, comprising the steps of holding an output signal of the solid state imaging device obtained in the second operation mode as a reference signal, and performing, in the first operation mode, correction processing of the output signal of the solid state imaging device using the reference signal.
In the signal processing method, the output signal of the solid state imaging device obtained in the second operation mode is held as a reference signal for cancellation of vertical string-like fixed pattern noises generated from the horizontal scanning circuit. Then, in the first operation mode, the reference signal stored and held is subtracted from the output signal of the solid state imaging device to cancel the vertical string-like fixed pattern noises generated from the horizontal scanning circuit.
With the signal processing method for a solid state imaging device, since an output signal obtained in the second operation mode from the solid state imaging device is stored and held as a reference signal and, in the first operation mode, the reference signal is used to correct the output signal of the solid state imaging device, that is, to cancel fixed pattern noises of the output signal, vertical string-like fixed pattern noises generated from the horizontal scanning circuit and so forth can be canceled with certainty.
According to a further aspect of the present invention, there is provided a camera, comprising a solid state imaging device which includes a plurality of pixels arranged in rows and columns, a vertical scanning circuit for controlling control electrodes of the pixels in the same rows connected commonly by vertical selection lines, a horizontal scanning circuit for successively outputting pixel signals outputted through vertical signal lines, to which main electrodes of the pixels of the same columns, are connected commonly in units of a row, and an output circuit for outputting the pixels signals from the horizontal scanning circuit to the outside, and has a first operation mode in which the pixels are reset after the pixel signals are read out and a second operation mode in which the pixel signals are read out after the pixels are reset, an optical system for introducing incident light to an imaging area of the solid state imaging device, and a signal processing circuit for holding an output signal of the solid state imaging device obtained in the second operation mode as a reference signal and performing correction processing of the output signal of the solid state imaging device in the first operation mode using the reference signal.
In the camera, in the second operation mode of the solid state imaging device, a reference signal for cancellation of vertical string-like fixed pattern noises is obtained without being influenced by incident light or the like even if mechanical light interception means such as a shutter is not used. The reference signal is stored and held in the signal processing circuit. Then, in the first operation mode, the reference signal stored and held is subtracted from the output signal of the solid state imaging device by the signal processing circuit to cancel vertical string-like fixed pattern noises generated from the horizontal scanning circuit.
With the camera, in the second operation mode of the solid state imaging device, even if mechanical light interception means such as a shutter is not used, a reference signal for cancellation of vertical string-like fixed pattern noises generated from the horizontal scanning circuit is obtained without being influenced by incident light or the like, and in the first operation mode, correction processing for an output signal of the solid state imaging device is performed with certainty by the signal processing circuit using the reference signal. Consequently, an image output of a high picture quality can be obtained.
According to a yet further aspect of the present invention, there is provided a solid state imaging device, comprising a plurality of pixels arranged in rows and columns, first switch means provided for each of the columns and having a first terminal connected to a vertical signal line to which output terminals of the plurality of pixels are connected in units of a column, first and second storage means provided for each of the columns and having first terminals connected commonly to a second terminal of the first switch means, second and third switch means provided for each of the columns and connected between second terminals of the first and second storage means and a reference potential point, and a vertical output circuit provided for each of the columns and including a horizontal selection switch connected between the second terminal of the first storage means and a horizontal signal line.
According to a yet further aspect of the present invention, there is provided a driving method for a solid state imaging device which includes a plurality of pixels arranged in rows and columns, first switch means provided for each of the columns and having a first terminal connected to a vertical signal line to which output terminals of the plurality of pixels are connected in units of a column, first and second storage means provided for each of the columns and having first terminals connected commonly to a second terminal of the first switch means, second and third switch means provided for each of the columns and connected between second terminals of the first and second storage means and a reference potential point, and a vertical output circuit provided for each of the columns and including a horizontal selection switch connected between the second terminal of the first storage means and a horizontal signal line, comprising the steps of turning on, in a horizontal blanking period, the first switch means to sample a signal in a bright state while the second switch means is in an on-state and then turning off the first switch means to hold the signal in the bright state in the first storage means, turning off the second switch means and then turning on the third switch means which is in an off-state, turning on the first switch means again to sample a signal in a dark state and then turning off the first switch means again to hold the signal in the dark state into the second storage means, and turning on, in a horizontal effective period, the horizontal selection switch to read out a voltage on the output side of the first storage means into the horizontal signal line and then turning on the second switch means to read out a reference potential into the horizontal signal line.
In the solid state imaging device and the driving method for a solid state imaging device, in a horizontal blanking period, the first switch means is first turned on to sample a signal in a bright state after pixel resetting while the second switch means is in an on-state, and then the first switch means is turned off to hold the signal in the bright state in the first storage means. In this instance, noise components caused by the switching of the first switch means are superposed on the first storage means. Then, the second switch means is turned off. In this instance, since the input side of the first storage means is in a floating state, noise components caused by the switching of the second switching means are not superposed on the first storage means.
Thereafter, the third switch means is turned on, and then, the first switch means is turned on again to sample a signal in a dark state obtained by resetting the pixels or the like. Then, the first switch means is turned off to hold the sampled signal in the dark state into the second storage means. In this instance, since the second storage means is connected to the output side of the first switch means, similarly as in the case wherein the signal in the bright state is held, noise components caused by the switching of the first switch means are superposed on the second storage means.
As a result, on the output side of the first storage means, noise components arising from characteristic dispersions of circuits, which make a factor of vertical string-like fixed pattern noises superposed on the first and second storage means, that is, caused by the switching of the first switch means, are canceled, and besides, a reference potential is added to signal components from which a finite difference between the signal in the bright state and the signal in the dark state, that is, fixed pattern noises arising from characteristic dispersions of the pixels, have been removed and a resulting signal is outputted.
Thereafter, in a horizontal effective period, the horizontal selection switch is turned on to read out a voltage on the output side of the first storage means, that is, the signal obtained by adding the reference potential to the signal components (the finite difference between the signal in the bright state and the signal in the dark state), into the horizontal signal line. Then, the second switch means is turned on to read out the reference potential.
Consequently, the signal obtained by adding the reference potential to the signal components and the reference potential are transmitted successively in units of a column on the time base to the horizontal output circuit by the horizontal signal line. Then, as a finite difference between the signal obtained by adding the reference potential to the signal components and the reference potential is calculated, characteristic dispersions of the circuits between columns of the vertical output circuit on which the two signals are superposed commonly are canceled. As a result, a signal from which not only fixed pattern noises arising from characteristic dispersions of the pixels but also vertical string-like fixed pattern noises arising from characteristic dispersions of the circuits have been canceled is obtained.
In summary, with the solid state imaging device and the driving method for a solid state imaging device, since a circuit which reads out a signal voltage in a bright state and a signal voltage in a dark state from each pixel into a horizontal signal line is constructed such that read-out paths for the two signal voltages are made same as each other, not only fixed pattern noises arising from characteristic dispersions of the pixels but also vertical string-like fixed pattern noises arising from characteristic dispersions of circuits can be suppressed.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters.